Perovskite-Type Oxides
A number of ABO.sub.3 type perovskite-type oxides, wherein A is a trivalent element such as rare earth element, B is also a trivalent element such as Co, Mn, Fe, Ni, Al, Cr, Sc or noble metal and O is oxygen, are known in the prior art. A large number of variations of this perovskite-type oxides, obtained from a partial substitution at the site A by divalent element such as Sr, Ba and the like and/or at the site B by other trivalent element are also known. Also, the ABO.sub.3 -type perovskite oxides wherein A is a divalent element such as Ca, Ba, Sr or Pb and B is tetravalent element such as Ti, Zr, Sn or Mn, with or without the partial substitution at the site A by trivalent element, are known in the prior art. Similarly, a number of A.sub.2 BO.sub.4 -type perovskite oxides, wherein A is Ca, Sr or rare earth element and B is Ti, Zr or Cu, with or without partial substitution by other elements at the site A or at the site B or at both the sites, are known. (Ref. L. G. Tejuca et. al. Adv. Catal. 36(1989)237; E. J. Baran, Catal. Today 8(1990)133).
Perovskite-type oxides containing transition metals are good catalysts for the total oxidation of hydrocarbons and carbon monoxide and hence are useful for the CO and hydrocarbon emission control by their complete combustion (Ref. McCarty and Wise, Catal. Today 8(1990)231; Yamazoe and Teraoka, Catal. Today 8(1991)175; Seiyama, Catal. Rev.-Sci. Eng. 34(1992) 281 and B. Viswanathan, Catal. Rev.-Sci. Eng. 34(1992) 337). Perovskite-type oxides are also useful for a number of other catalytic processes, such as partial oxidation of hydrocarbons and oxygenates, hydrogenation and hydrogenolysis of hydrocarbons, hydrogenation of carbon oxides and decomposition of N.sub.2 O (Ref. T. Shimizu Catal. Rev.-Sci. Eng.34(1992)355; Ichimura et. al. Catal. Rev.-Sci. Eng.34(1992)301; Fierro, Catal. Rev.-Sci. Eng.34(1992)321; Swamy and Christopher, Catal. Rev.-Sci. Eng.34(1992)409).
Several methods based on the precursor preparation from mixed oxides, freeze-drying, spray drying, co-precipitation and sol-gel, followed by thermal decomposition of the precursor and/or thermal reaction between metal oxides are known in the prior art for the preparation of perovskite-type oxides. However, because of the involvement of high reaction temperature in the formation of perovskite-type oxides, the surface area of the resulting perovskite-type oxides is rather low. For example, the surface areas of the perovskite-type oxides obtained from mixed oxide technique is below 2 m.sup.2 g.sup.-1 and the surface area of perovskite-type oxides obtained from coprecipitation technique are in the range of 1 to 10 m.sup.2 g.sup.-1. Perovskite-type oxides having surface area upto 20 and 30 m.sup.2 g.sup.-1 could be prepared by the prior art processes based on the spray drying and freeze-drying, respectively, of mutually soluble compounds and thereby achieving good chemical homogeneity for perovskite precursors, which are fired at lower temperatures, to get high surface area perovskite-type oxides. However, the use of these processes are limited because of the requirements of mutually soluble compounds and specialized equipments and therefore, the cost of perovskite-type oxide prepared by these processes is high (Ref. Voorhoeve et. al. Science 195(1977)827-833; E. J. Baran, Catal. Today 8(1990)133).
Because of a very slow diffusion of metal cations and O.sup.2- anions in solid state, the formation of perovskite-type oxide from non-homogeneously mixed metal oxides requires high temperatures and long reaction period and hence the resulting perovskite-type oxide has a low surface area due to sintering or crystal growth. The temperature for the formation of perovskite-type oxide can however be reduced and thereby the surface area of the perovskite-type oxide can be increased if a homogeneously mixed metal oxides or compounds, which on decomposition or calcination are converted into metal oxides, are used as precursors for the preparation of perovskite-type oxides. But, in practice, it is very difficult to prepare such homogeneously mixed metal oxides, hydroxides or other compounds, which on decomposition or calcination are converted into metal oxides with homogeneous distribution of metal cations.
For perovskite-type oxides to be more active as catalysts, their surface area must be increased. The perovskite-type oxides deactivated due to sintering or crystal growth in the high temperature catalytic processes or other high temperature processes also need to be regenerated by increasing their surface area by some means. Hence there is a great need for increasing the surface area of the perovskite-type oxides, after their preparation by the processes known in the prior art, and thereby increasing the catalytic activity of the perovskite-type oxides. There is also a need of an invention for regenerating the perovskite-type oxides, deactivated due to sintering or crystal growth during catalytic processes, by increasing their surface area and thereby regaining the lost catalytic activity of the perovskite-type oxides.
Accordingly, the primary object of the present invention is to increase the surface area and/or catalytic activity of perovskite-type oxides which have low surface area due to their formation at high temperature or because of their sintering or crystal growth during their use, could be increased.
Thus, it is an object of the present invention to provide a novel process for the activation of perovskite-type oxides, which have low surface area or the surface area of which is decreased due to sintering or crystal growth, so that their surface area is increased.
Another important object of this invention is to provide a novel process for the activation of perovskite-type oxides, so that their catalytic activity is increased.